Slide tray indexing mechanism for slide projectors or the like

ABSTRACT

A forward-reverse slide tray indexing mechanism is provided which comprises a two-pronged fork-like plate slidably and pivotally mounted on the base of a slide projector and having a pawl at the end of each prong. A slide carrier is adapted to engage the plate and upon reciprocating movement thereof to push one of the pawls into engagement with an indexing gear to cause incremental rotational movement of the gear and a corresponding rack of teeth on a slide tray to advance the tray to the next slide position. Forward and reverse operation is controlled by pivotally moving one or the other of the pawls into position for engagement with the indexing gear. A pair of guide brackets disposed adjacent the indexing gear are adapted to be engaged by the pawl not in use, and pivotally move the plate into a neutral position so that both pawls are out of engagement with the indexing gear at the end of each withdrawal stroke of the slide carrier.

llnited States Patent [1 1 Denis et al.

[ Oct. 29, 1974 SLIDE TRAY INDEXING MECHANISM FOR- SLIDE PROJECTORS OR THE LIKE [73] Assignee: GAF Corporation, New York, N.Y.

[22] Filed: Oct. 27, 1971 [21] Appl. No.: 192,927

[30] Foreign Application Priority Data Oct. 28, 1970 Belgium 95684 [52] US. Cl. 353/116 [51] Int. Cl. G03b 23/04 [58] Field of Search 353/103-116, 353/117; 40/79 [56] References Cited UNITED STATES PATENTS 2,874,497 2/1959 Huff 353/116 3,110,120 11/1963 Danner 40/79 3,165,971 l/1965 Zillmer 353/106 3,263,559 8/1966 Golden 40/79 3,690,186 9/1972 Fleissncr 353/103 Primary Examiner-William D. Martin, Jr.

Assistant Examiner-A. Jason Mirabito Attorney, Agent, or Firm-Walter C. Kehm; Samson B. Leavitt [57] ABSTRACT A forward-reverse slide tray indexing mechanism is provided which comprises a two-pronged fork-like plate slidably and pivotally mounted on the base of a slide projector and having a pawl at the end of each prong. A slide carrier is adapted to engage the plate and upon reciprocating movement thereof to push one of the pawls into engagement with an indexing gear to cause incremental rotational movement of the gear and a corresponding rack of teeth on a slide tray to advance the tray to the next slide position. Forward and reverse operation is controlled by pivotally moving one or the other of the pawls into position for engagement with the indexing gear. A pair of guide brackets disposed adjacent the indexing gear are adapted to be engaged by the pawl not in use, and pivotally move the plate into a neutral position so that both pawls are out of engagement with the indexing gear at the end of each withdrawal stroke of the slide carrier.

12 Claims, 4 Drawing Figures BACKGROUND OF THE INVENTION It is well known to provide slide projectors of the type adapted to receive a slide tray containing a plurality of uniformly spaced apart slides and having a slide carrier which is reciprocal in a direction transversely of the optical axis of the projector for advancing slides from the tray to the viwing position and then returning such slides to the slide tray.

Such projectors are often provided with means for indexing the slide tray from one slide position to the next so that all the slides in the tray may be sequentially advanced into the viewing position. Many projectors utilize slide trays which are provided with a peripheral rack of teeth or other projections, each one of which corresponds to a slide receiving space in the tray. An indexing gear in the projector is adapted to operatively engage the teeth or projections to sequentially advance the tray from slide to slide. The indexing means associated with such projectors are often in the form of a pawl type device which is slidably and pivotally mounted on the base of the projector and urged into engagement with the indexing gear by means of the slide carrier. In many cases a pair of alternate pawl plates are provided to accomplish forward and reverse movement of the indexing gear. One pawl is adapted to engage the side of the indexing gear for rotation in one direction, while the other pawl is alternately adapted to engage the other side of the indexing gear for rotation in the opposite direction. The pawls are pivotally mounted on the end of each plate and are provided with springs which strap the pawl out of engagement with the gear at the completion of the slide changing stroke. A ball type detent is usually employed to engage the indexing gear to maintain it in the proper position to hold the slide tray at the end of each slide transfer stroke. An indexing mechanism of this type is shown in U.S. Pat. No. 3,120,150 to Brown.

Unfortunately, this type of device is rather complex and relatively expensive to construct. In addition, it has a disadvantage of being noisy and unpleasant due to the snapping of the pawls and the locking of the ball type detent between the teeth of the indexing gear during successive rotations of the gear.

Other forward-reverse indexing means which have been provided comprise a single reciprocable pawl plate having a pair of fork-like pawl projections. The plate is adapted to be pivoted so that either one of the pawls may engage the indexing gear.

An example of the fork-like indexing mechanism is disclosed in British Pat. No. 1,048,079. Although the construction shown in the British patent is simpler and less expensive to construct than the double pawl system described above. it also has several drawbacks. The fork is pivoted into the desired position by means of a solenoid and an appropriate connecting member,and is held in that position throughout the reciprocating stroke of the slide carrier, so that when the slide carrier is fully withdrawn and the tray has been advanced to the next slide position, one of the pawls on the indexing plate will maintain its engagement with the teeth of the indexing gear so as to lock the gear and the tray in place. In order to mannuallyadvance the tray to another slide position without utilizing the slide carrier the entire tray must be removed from the projector to be disengaged from the indexing gear. This of course is quite inconvenient to the operator, and a severe disadvantage of the device.

SUMMARY OF THE INVENTION In accordance with the present invention, a slide projector indexing mechanism is provided which overcomes the deficiencies found in prior devices. The indexing mechanism of the present invention utilizes a fork-like slidable pawl plate for forward and reverse indexing operation, but is arranged such that at the end of its indexing stroke neither pawl is in engagement with the indexing gear of the projector so that the tray might be manually moved to another slide position without removing the same from the slide projector. In addition, the present invention provides means to secure the indexing gear in the selected position to avoid accidental movement or misalignment of the tray with out preventing manual movement thereof when the slide changer is in the withdrawn position after returning a slide to the tray. Furthermore, the slide tray indexing mechanism of the invention is extremely quiet in operation, relatively simple in construction and rather inexpensive to produce.

in general, the slide tray indexing mechanism of the invention comprises, in combination, a two-pronged fork like plate having a pawl at the end of each prong slidably and pivotally mounted on the base of a slide projector for reciprocating movement transversely to the direction of movement of the slide tray; means on the plate for operatively engaging the slide carrier of the projector upon reciprocating movement of the slide carrier to move the plate into engagement with the indexing gear of the projector to cause rotational movement of said gear through an arcuate extent corresponding to the distance between two adjacent slides in the slide tray, forward and reverse control means disposed in operative engagement with said pawl plate and adapted to alternately pivot said plate into a forward position in which one of said pawls engages the indexing gear for rotation in one direction and a reverse position in which the other of said pawls engages the indexing gear for rotation in the opposite direction; and guide means in the form of brackets disposed adjacent the indexing gear and adapted to be engaged by the pawl plate to pivotally move said plate into a neutral position with both pawls out of engagement with the indexing gear at the end of each tray advancement stroke with the slide corner in its withdrawn position. It is an important feature of the invention that the pawl plate terminates its stroke in the neutral position out of engagement with the indexing gear of the projector. This feature permits manual movement of the slide tray within the slide receiving groove of the projector without cycling the slide carrier. To accomplish this, in addition to the guide brackets the forward-reverse control means is adapted to temporarily disengage the pawl plate as it nears the end of its stroke. In this manner, the

pawl is free to pivot into its neutral position out of engagement with the indexing gear upon its engagement with the guide means without interference from the forward-reverse control means.

To prevent unintentional movement of the slide tray when the slide carrier is fully withdrawn and the pawl plate is in its neutral position, a detent means in the form of a spring clip engages the teeth of the indexing gear to maintain it in the selected position. The spring clip, however, does not interfere with the rotational movement of the gear when force is applied to it by means of the manual movement of the slide tray or by the pawl plate during normal reciprocating movement thereof.

The guide means for moving the pawl plate into its neutral position are designed such that the plate is gradually pivoted out of engagement with the indexing gear rather than snapped into place as is inherent in many of the prior slide projectors. This accounts for a much quieter operation which is more pleasant to the operator of the projector.

The forward-reverse control means is preferably in the form of a bracket which extends transversely of the pawl plate and has upstanding tab portions adapted to engage the sides of the pawl plate. The bracket is slidable between a forward position in which one of the tabs engages one side of the pawl plate to push it into a position for forward operation, and a reverse position in which the other tab engages the opposite side of the pawl plate to move it into a position for reverse operation. The bracket can be controlled by an operating lever or knob connected thereto and disposed on the exterior of the projector for manual selection of the forward or reverse positions, or by an electromagnetic solenoid device operated by a selector switch.

These and other features of the indexing means of the invention are more fully described with reference to the annexed drawings.

BRIEF DESCRIPTION OF THE DRAWlNGS F IG. 1 is a perspective view of a slide projector embodying the invention.

HO. 2 is a partial cross-section transverse to the optical axis of the projector taken along the lines 22 of FIG. 1.

FIG. 3 is a top plan view of the slide indexing mechanism of the invention.

FlG. 4 shows schematically various positions of the pawl plate of the indexing means.

As shown in FIG. 1 the preferred embodiment of the projector of the present invention includes a base plate 1 and a projector housing 2 fitted thereover. The housing 2 is provided with a slide tray receiving channel or trough-like groove 3 which extends along one side thereof and is open at its top and at its forward and rear ends for receiving a known type slide tray 6 in which a plurality of slides 7 may be stored.

As shown in H0. 2 the slide tray 6 is formed of an upper wall 8 and a lower wall 9 connected together by transverse partitions l uniformly spaced from each other by a distance approximately equal to the thickness of the frame 11 of slides 7. Although a box-like slide tray is shown. it will be apparent to those skilled in the art that projectors of this type are also adapted to receive circular vertically disposed slide trays such as that shown in US. Pat. No. Re. 26,6]9 to Hall.

The optical axis of the projector is indicated by a broken line XX in FIG. 1, and as is well known in the art, lamp. condenser lenses and focusing lenses are disposed in the projector housing 2 in alignment with the optical axis XX thereof. A vertically extending slide receiving aperture 12 is formed in vertical wall 13 which defines the inner wall of slide receiving groove 3, approximately midway between the front wall 14 and the rear wall 15 of the projector housing 2, to permit LII the transfer of slides from the slide tray disposed in the groove 3 to a viewing position in the projector and the return of a slide from the viewing position to the slide tray.

A slide carrier 16 is mounted for reciprocable sliding movement transversely of the optical axis XX and is adapted for transferring slides between the slide tray 6 and the viewing aperture 12 in the projector. The slide carrier 16 is characterized by an elongated bar member 17 supported for sliding movement by guide blocks 18, a handle 17a which extends upwardly from the outer end of the bar member 17, a pusher arm 19 which extends inwardly from the upper end of the handle mem ber towards the optical axis XX of the projector in parallel spaced arrangement above the bar member 17, and by a vertically disposed plate member (not shown) which is secured by suitable fastening means to the opposite end of bar member 17. Details of the slide carrier of the type embodied in the projector disclosed herein are more fully described in US. Pat. No.

3,183,774 to Kurz.

When the slide carrier 16 is moved inwardly from its outermost position, the innermost end of the pusher arm 19 engages a slide stored in the slide tray 6 so as to transfer the slide from the slide tray through the vertical slide aperture 12 to a viewing position in the projector on the optical axis. The slide is returned to the slide tray 6 from its viewing position in the projector upon outward movement of the slide carrier 16 from its innermost position. After the slide carrier has returned the previously viewed slide to the slide tray 6, the slide tray is automatically advanced by the indexing means of the invention to present the next slide in the tray for viewing.

The slide trays employed are provided with a peripheral rack of teeth 21, or other suitable projections, each of which corresponds to a slide receiving space in the tray. At the base of the tray receiving groove 3 in the projector an indexing gear 20 is disposed to operatively engage the teeth 21 to sequentially advance the slide tray. The indexing gear 20 is rigidly connected to a coaxial pinion gear 22 for rotation therewith by means of a vertical axle 23 and a hold-down member 24 fastened to the base frame 1.

The indexing mechanism of the invention comprises a fork-like pawl plate 25 slidably mounted for reciprocating and pivotal movement on the base 1 of the slide projector. The plate 25 has a reverse pawl 26 shown in the operative position in FIG. 3 and a forward pawl 27, both of which alternately engage the teeth 33 of pinion 22 in their respective operating positions.

The plate 25 has at its rear portion an axial slot 28 which is engaged by a mounting pin 29 to permit pivotal movement of the plate 25 and limit the reciprocating movement thereof. A retention disk 30 is attached to the pin 29 which guides plate 25 along the base plate 1. At the approximate midpoint of plate 25, a rectangular opening is provided which receives a positioning pin 32, adapted to limit the pivotal movement of plate 25 between the position shown in FIG. 3 for reverse movement whereby pawl 26 engages teeth 33 of pinion 22 to effectuate counter-clockwise rotation thereof, and a forward position in which pawl 27 engages the teeth 33 of pinion 22 to effectuate clockwise rotation thereof. A second retention disk 34 is attached to the top of positioning pin 32 to further guide the plate.

A C-shaped forward-reverse activating bracket 35 is slidably disposed beneath plate for reciprocating movement in a plane substantially perpendicular thereto. The bracket is formed with a pair of upstanding posts 37 and 38, adapted to selectively engage opposite sides of the pawl plate 25 to pivotally move the plate about mounting pin 29 into a position for either forward or reverse operation. The C-shaped bracket 35 can be connected by suitable linking means to an electromagnetic solenoid (not shown) to move the plate into the desired position. A forward-reverse selector switch in the electrical circuit to the solenoid can be mounted on the projector housing or remote hand piece to permit manual actuation of the solenoid.

Alternatively, the bracket 35 can be mechanically connected to an external selector knob or lever mounting on the'housing for manual selection of the desired anod of operation.

When the reverse slide tray direction is selected bracket 35 is moved into the position shown in FIG. 3.- ln moving toward this position, upstanding post 37 of the bracket urges plate 25 to pivot about mounting pin 29 so that the reverse pawl 27 will be in a position to operatively engage the teeth 33 of indexing pinion 22. When the forward position is selected, bracket 35 is moved in the opposite direction so that upstanding post 38 engages the opposite side of plate 25 and moves it into a forward operating position in which pawl 27 is adapted to operatively engage the teeth of the indexing pinion 22.

The pawl plate 25 is cyclically driven back and forth to engage and rotate the indexing pinion by the opera tive engagement therewith of the slide carrier mechanism 16. An upstanding tab portion 39 formed on plate 25 is provided to effectuate engagement with corresponding downwardly extending projections 41 and 42 on the bar 17 of the slide carrier mechanism. Upon withdrawal of the slide carrier from the slide projector to transfer a slide from the projection gate to the slide tray 6, projection 41 engages tab 39 to slidably move the pawl plate into engagement with the indexing pinion 22 to provide stepped rotation thereof to index the slide tray to the next slide position. When the slide carrier is once again moved into a position to transport the next slide into the projection gate, projection 42 on the bar 17 engages the opposite side of upstanding tab 39 to return the pawl plate to its starting position. in order to permit a precise adjustment of the relative positions of the projection 41 and 42 and of the upstanding tab 39, to ensure that a slide is returned to the tray prior to the engagement of the indexing pinion by the pawl plate, the latter is preferably formed as a part of a support fastened to the plate 25 by means of a screw 46. if desired, the projection 42 can be eliminated and a spring provided to return the plate to its starting position.

The pawl plate 25 is adapted to complete each indexing stroke in a neutral position as shown by the dashed lines in FIG. 4. In this position, pawls 26 and 27 are out of engagement with indexing pinion 22, and thus permit the manual movement of the slide tray 6 within the slide receiving channel 3 without the necessity of lifting the tray out of engagement with the indexing gear 20. Undesired movement of the pinion 22, however, is prevented by a detent 48 in the form of a spring clip mounted adjacent pinion 22, and adapted to engage the teeth 33 thereof to hold the gear in the selected position. Plate 25 is guided into the neutral position at the completion of its stroke by angularly disposed guide brackets 44 and 45 mounted adjacent pinion 22. As the plate begins its stroke, the pawl not in engagement with the gear, i.e. pawl 27 shown in FIG. 3, engages the corresponding bracket 45. Movement of the pawl against the bracket causes plate 25 to gradually pivot about pin 29 and terminate its stroke in the neutral position. Cutout portions 43 are provided on both sides of plate 25 to permit the disengagement of upstanding posts 37 and 38 as plate 25 commences its stroke to permit gradual movement into the neutral position.

FIG. 4 shows schematically three successive positions of the pawl plate 25. The solid line path corresponds to the reverse position of the pawl plate 25 prior to engagement with the teeth 33 of the indexing pinion 22, the dot-dash line represents an intermediate position of the pawl plate 25 during engagement with the pinion 22, and the dashed line corresponds to the neutral position of pawl plate 25 at the end of its indexing stroke.

Several alternative embodiments within the scope of the invention can be provided. For example, a pair of upstanding tabs can be formed on the pawl plate and a single, downwardly extending projection can be formed on the slide carrier to provide the drive linkage between the two.

As'an alternative to the two upstanding posts 37 and 38 provided on the C-shaped bracket 35 a single stud displaceable within the opening of pawl plate 25 can also be utilized. Likewise, the positioning pin 32 can be replaced by two pins positioned on the outside of pawl plate 25 to prevent excessive pivotal movement.

In operation, the slide carrier 16 is withdrawn to its outermost position as shown in FIG. 2 and a slide tray is placed in the slide receiving groove 3. The slide carrier can then be reciprocated to transport a slide 7 to the projection gate. If it is desired to view the next slide in the tray, the C-shaped bracket 35 is set in the forward position to pivotally move pawl plate 25 into a position whereby pawl 27 will engage the indexing pinion 22 to incrementally rotate the pinion in a clockwise direction upon withdrawal of the slide carrier. The engaging tab 39 on the pawl plate the the projection 41 on the bar 17 are positioned such that the previously viewed slide is returned to the slide tray prior to the engagement of the pawl and the indexing pinion. The incremental rotation of pinion 22 causes indexing gear 20 to rotate the same distance thereby transmitting the rotational movement to the tray rack 21 to advance the tray the distance of one slide. As the pawl plate nears the end of its indexing stroke pawl 26 engages the guide bracket 44 to gradually pivot the pawl plate so that upon the completion of the indexing stroke neither pawl is in engagement with the indexing pinion. However, the indexing gear and the tray are firmly held in the selected position by the spring detent 48.

If the previously viewed slide is to be viewed again, the bracket 35 is set in the reverse position to pivotally move the pawl plate so that pawl 26 will engage the indexing pinion to incrementally rotate the same in the counter-clockwise direction to back-up the slide tray the distance of one slide upon withdrawal of the slide carrier. In all other respects the reverse operation is the same as the forward operation.

Modifications of the indexing mechanism within the scope of the invention will be apparent to those skilled in the art.

What is claimed is:

1. In a slide projector of the type adapted to receive a slide tray having a plurality of slide receiving spaces, a base, and a slide carrier slidably disposed above the base for reciprocating movement transversely to the slide tray for transferring a slide from the slide tray to the viewing position on the optical axis of the projector and returning the same to the slide tray, a reversible slide tray indexing mechanism for advancing the slide tray to sequentially present each slide contained in the slide receiving spaces for viewing comprising, in combination, an indexing gear rotatably disposed on the projector and adapted to engage a corresponding rack of projections formed on the periphery of the slide tray; a two-pronged, fork-like unitary plate having a pawl at the end of each prong slidably and pivotally mounted on the base of the projector for reciprocating movement transversely to the direction of movement of the slide tray; means on the plate for operatively engaging the slide carrier upon reciprocating movement of said slide carrier to move the plate into engagement with the indexing gear to cause incremental rotational movement of said gear through an arcuate extent corresponding to the distance between two adjacent slide receiving spaces in the slide tray; a slidable forward and reverse control bracket transversely disposed with respect to and in operative engagement with said pawl plate and having tabs on opposite sides of the plate adapted to selectively contact and pivot said plate into a forward position in which one of said pawls engages the indexing gear for rotation in a direction such that the slide tray is advanced to the next slide receiving space, or a reverse position in which the other of said pawls engages the indexing gear for rotation in the opposite direction to back up the slide tray to the previous slide receiving space; and a pair of guide brackets disposed adjacent the indexing gear and adapted to be engaged by either one of the pawls of said pawl plate to pivotally move said plate into a neutral position in which both pawls disengage the indexing gear at the end of each tray advancement stroke.

2. An indexing mechanism according to claim 1, in which the guide brackets are angularly disposed on opposite sides of the indexing gear.

3. An indexing mechanism according to claim 1, further including detent means to secure the indexing gear in the desired position.

4. An indexing mechanism according to claim 1, in which the pawl plate has an elongated slot formed therein; and is slidably secured directly to the base of the projector by means of a mounting pin which extends through said slot.

5. An indexing mechanism according to claim 1, in which the pawl plate has a rectangular opening formed at an intermediate point therein; and a positioning pin extends through said opening and alternately engages the sides thereof to limit the pivotal movement of said late.

p 6. An indexing mechanism according to claim 1, in

which the pawl plate has a recessed portion on each side thereof which is adapted to alternately receive the tabs of the forward-reverse control bracket to permit pivotal movement of said plate into the neutral position upon engagement of said plate and said guide means.

7. An indexing mechanism according to claim 1, in which the means on the pawl plate for operatively engaging the slide carrier comprise an upstanding tab which is contacted by a corresponding series of projections downwardly extending from the slide carrier, the position of the upstanding tab and the downwardly extending projections being such that upon withdrawal of the slide carrier to return a slide from the viewing position to the slide tray, said slide is positioned in the tray prior to the engagement of the pawl plate and the indexing gear.

8. An indexing mechanism according to claim 1, further including biasing means to return the pawl plate to its starting position when a slide is being transported by the slide carrier to the viewing position.

9. An indexing mechanism according to claim 1 in which the guide brackets are adapted to engage the pawl of the pawl plate which is not in position to' engage the indexing gear, thus pivoting the opposite pawl out of engagement with said gear.

10. A reversible indexing mechanism for incrementally advancing a pinion type gear mounted on a base member comprising, in combination, a two-pronged fork-like plate having a pawl at the end of each prong slidably and pivotally mounted on a base member disposed adjacent said gear; means for reciprocating said plate into and out of engagement with said pinion to cause incremental rotational movement thereof through an arcuate extent corresponding to the distance between adjacent teeth; a slidable forward and reverse control bracket transversely disposed with respect to the pawl plate having tabs on opposite sides of the said plate adapted to selectively engage each of said opposite sides to pivotally move said plate into a forward position in which one of said pawls engages the gear for rotation in one direction, and a reverse position in which the other of said pawls engages the gear for rotation in the opposite direction; and a pair of guide brackets disposed adjacent the gear and adapted to be engaged by either one of the pawls of the pawl plate at the end of each indexing stroke to pivotally move said plate into a neutral position with both pawls out of engagement with said gear.

11. An indexing mechanism according to claim 10, in which the gear is adapted to engage a corresponding rack of teeth formed on the periphery of a slide receiving tray for use in slide projectors, whereby the incremental rotation of said gear advances said slide tray to sequentially present each slide for viewing.

12. An indexing mechanism according to claim 11, in which the means for reciprocating the pawl plate comprises a slide carrier adapted to transport the slides from the slide tray to the viewing position, and to return the slides to said tray after viewing. 

1. In a slide projector of the type adapted to receive a slide tray having a plurality of slide receiving spaces, a base, and a slide carrier slidably disposed above the base for reciprocating movement transversely to the slide tray for transferring a slide from the slide tray to the viewing position on the optical axis of the projector and returning the same to the slide tray, a reversible slide tray indexing mechanism for advancing the slide tray to sequentially present each slide contained in the slide receiving spaces for viewing comprising, in combination, an indexing gear rotatably disposed on the projector and adapted to engage a corresponding rack of projections formed on the periphery of the slide tray; a two-pronged, fork-like unitary plate having a pawl at the end of each prong slidably and pivotally mounted on the base of the projector for reciprocating movement transversely to the direction of movement of the slide tray; means on the plate for operatively engaging the slide carrier upon reciprocating movement of said slide carrier to move the plate into engagement with the indexing gear to cause incremental rotational movement of said gear through an arcuate extent corresponding to the distance between two adjacent slide receiving spaces in the slide tray; a slidable forward and reverse control bracket transversely disposed with respect to and in operative engagement with said pawl plate and having tabs on opposite sides of the plate adapted to selectively contact and pivot said plate into a forward position in which one of said pawls engages the indexing gear for rotation in a direction such that the slide tray is advanced to the next slide recEiving space, or a reverse position in which the other of said pawls engages the indexing gear for rotation in the opposite direction to back up the slide tray to the previous slide receiving space; and a pair of guide brackets disposed adjacent the indexing gear and adapted to be engaged by either one of the pawls of said pawl plate to pivotally move said plate into a neutral position in which both pawls disengage the indexing gear at the end of each tray advancement stroke.
 2. An indexing mechanism according to claim 1, in which the guide brackets are angularly disposed on opposite sides of the indexing gear.
 3. An indexing mechanism according to claim 1, further including detent means to secure the indexing gear in the desired position.
 4. An indexing mechanism according to claim 1, in which the pawl plate has an elongated slot formed therein; and is slidably secured directly to the base of the projector by means of a mounting pin which extends through said slot.
 5. An indexing mechanism according to claim 1, in which the pawl plate has a rectangular opening formed at an intermediate point therein; and a positioning pin extends through said opening and alternately engages the sides thereof to limit the pivotal movement of said plate.
 6. An indexing mechanism according to claim 1, in which the pawl plate has a recessed portion on each side thereof which is adapted to alternately receive the tabs of the forward-reverse control bracket to permit pivotal movement of said plate into the neutral position upon engagement of said plate and said guide means.
 7. An indexing mechanism according to claim 1, in which the means on the pawl plate for operatively engaging the slide carrier comprise an upstanding tab which is contacted by a corresponding series of projections downwardly extending from the slide carrier, the position of the upstanding tab and the downwardly extending projections being such that upon withdrawal of the slide carrier to return a slide from the viewing position to the slide tray, said slide is positioned in the tray prior to the engagement of the pawl plate and the indexing gear.
 8. An indexing mechanism according to claim 1, further including biasing means to return the pawl plate to its starting position when a slide is being transported by the slide carrier to the viewing position.
 9. An indexing mechanism according to claim 1 in which the guide brackets are adapted to engage the pawl of the pawl plate which is not in position to engage the indexing gear, thus pivoting the opposite pawl out of engagement with said gear.
 10. A reversible indexing mechanism for incrementally advancing a pinion type gear mounted on a base member comprising, in combination, a two-pronged fork-like plate having a pawl at the end of each prong slidably and pivotally mounted on a base member disposed adjacent said gear; means for reciprocating said plate into and out of engagement with said pinion to cause incremental rotational movement thereof through an arcuate extent corresponding to the distance between adjacent teeth; a slidable forward and reverse control bracket transversely disposed with respect to the pawl plate having tabs on opposite sides of the said plate adapted to selectively engage each of said opposite sides to pivotally move said plate into a forward position in which one of said pawls engages the gear for rotation in one direction, and a reverse position in which the other of said pawls engages the gear for rotation in the opposite direction; and a pair of guide brackets disposed adjacent the gear and adapted to be engaged by either one of the pawls of the pawl plate at the end of each indexing stroke to pivotally move said plate into a neutral position with both pawls out of engagement with said gear.
 11. An indexing mechanism according to claim 10, in which the gear is adapted to engage a corresponding rack of teeth formed on the periphery of a slide receiving tray for use in slide projectors, whereby the incremental rotatIon of said gear advances said slide tray to sequentially present each slide for viewing.
 12. An indexing mechanism according to claim 11, in which the means for reciprocating the pawl plate comprises a slide carrier adapted to transport the slides from the slide tray to the viewing position, and to return the slides to said tray after viewing. 